The neural substrates of the processing of speech sounds /

This work includes several studies exploring functional specialization of human primary and anterior secondary auditory cortex (AC) in the two hemispheres. It was hypothesized that the left hemisphere prepotency for linguistic processing is based, in part, on specialized mechanisms within this hemisphere for the processing of acoustic transients relevant to speech. Evidence supporting this idea was obtained in a positron emission tomography study (PET). Since nonlinguistic stimuli were used, the observed left-hemispheric activation cannot be specific to the speech system, but must reflect a more general processing mechanism. A reanalysis of data from six auditory PET studies revealed that the peak focus of auditory activation was significantly posterior to Heschl's gyrus (HG) in the right hemisphere, while it encompassed HG in the left. The cause of this asymmetry is unknown, but it appears to hold true for other auditory functional imaging studies in the literature. Functional specialization in auditory regions was also explored by testing patients with anterior temporal-lobe resections from either the left (LT) or right (RT) hemisphere and normal volunteers on several linguistic and nonlinguistic auditory tasks. The excisions in these patients always included some secondary AC, and sometimes included primary AC (HG). I had speculated that anterior. secondary AC in the left hemisphere was specialized for processing word-sounds, but patients with excisions in this area were unimpaired in their ability to use such information to retrieve items from the mental lexicon. Furthermore these patients did not show a disproportionate impairment on a lexical decision task, when items were presented aurally instead of visually, compared to normal subjects. With few exceptions, a battery of psychophysical tests of auditory processes were performed normally by all groups. The notable exception was an impairment in the group of RT patients with lesions that included HG on a t